Simple changes to the format of a cell include changing the alignment of the characters, changing the font type or colour, changing the border, and changing the colour or pattern of the background.
The Alignment, Font, Border, and Background tabs are simple to understand simply by playing around with the settings and looking at the effect on a cell which contains text.
The Protection and Validation tabs are advanced functionality which you can ignore at the beginning. For explanations of these tabs, see the advanced description in Section 5.10.3.2 ― Validation Tab.